This invention relates to electric power transmission and, more particularly, to an auxiliary support for a horizontal line-post insulator which positions an electrical conductor relative to a supporting tower.
In the transmission of electrical power, towers or poles are used to carry electrical conductors high above the ground. In one type of construction, the conductors are spaced from a tower by horizontal line-post insulators which are fabricated, particularly, of a series of porcelain insulator sections.
In one form of this construction, the electrical conductors are connected to the outboard ends of the respective insulators by a structure, which may be referred to as a basket, which is secured at the outer end of the line-post insulator and includes a loop which encircles the conductor. The insulator is provided with a clamp that is tightly secured to the conductor and is attached by means of shear pins to the basket. In service, if loads exceeding the design loads plus a safety factor are applied to the insulator, the strength of the insulator may be exceeded and the insulator or its mountings to the tower may be damaged. To protect against such cause of damage, the shear pins fracture at a predetermined magnitude of force, so as to free the clamp and allow the conductor with the clamp to slide freely through the loop of the basket. Such an arrangement has proven satisfactory in protection of the line-post insulator under a variety of service overloads.
However, a hazard still exists in that vertical overloads, which include the weight of the cable, may be sufficiently strong so as to break the line-post insulator. While the foregoing arrangement of the shear pins is useful in the protection of horizontally directed forces, the arrangement may not provide a sufficient level of protection against vertically directed forces under some conditions. As a result, there have been some failures of power transmission systems wherein, after the breakage of one line-post insulator, the additional weight of the electrical conductor resulting from the loss of the line-post insulator is then transferred to the insulator of a neighboring tower which then also fails. Thus a series of failures may develop in the manner of a cascade in which many line-post insulators are damaged, and wherein the electrical conductor may fall to the ground. In the design of transmission lines, distances between points of support (span length) may be limited to ensure that conductor vertical loads on the insulators do not exceed the strength of the insulators for the given number of assumed insulator failure points. In addition points of conductor longitudinal restraint (dead ends) may be located so as to contain an occurring cascade failure and thus limit the event damage.
However, for existing transmission lines the above remedies are not practical due to expense and space limitations. A further problem arises in that, should it be desired to alter the construction of the towers in a transmission system so as to afford additional support to the insulators, some means would have to be developed for maintaining the supply of electric power to the users of such power during the refurbishment of the towers.